I'd say this is what makes a vladimir a Vlad. This skill allows you to harass, spell vamp off enemies. Try your best to use this skill as often as you can in lanes on enemy champions. I don't suggest using this skill to last hit during the early parts of the game because of it's 10 second cool down.
I take an early point in this skill only for its effect. This skill practically makes you invulnerable for two seconds which oftentimes will get you out of sticky situations in lane. I'd say try to time it for there CC moves if you use this skill too early, they will just wait it out and apply there CC to you right after you come out of it. Keep in mind that this skill cost 20% and has a 26 second CD at level 1. If your forced to use this skill, stay passive until it comes off CD. ALWAYS USE THIS BEFORE YOU FLASH!
Do not use this skill to farm early game, because of the amount it costs to keep it up. Only use it if you know you can Q an enemy right after. This skill costs 25% extra HP for 25% more damage. Late game always try to keep this at 4 stacks! Use it every time its up in team fights. One mistake that I see a lot of people doing is that when ther 1/4th HP they still use this skill to try to do damage, DO NOT DO THIS, YOU WILL KILL YOURSELF!
Try to place this skill on as much people as you can as early as you can in a team fight, this amplifies damage by 12% which makes a lot of a difference in a team fight. Don't forget that after 5 seconds it also does magic damage so no need to waste your flash to try to finish someone off! Combine your ignite with this for MAXIMUM damage!